The present invention relates to calibration structures for probing devices, and more particularly to improved calibration structures for suppressing undesirable electromagnetic modes resulting from the substrate of the calibration structure.
Coplanar transmission structures, such as coplanar waveguides, coplanar striplines, coplanar slotlines, and the like, are used in a wide variety of electronic applications. For example, coplanar waveguides are used in probes suitable to probe semiconductors at multi-gigahertz frequencies, such as described in U.S. Pat. No. 4,697,143. The probe described in the '143 patent has an approximately triangular shaped alumina substrate on which is formed a coplanar waveguide that tapers toward the point of the triangle. Bulk microwave absorbing material containing iron or ferrite and having a high magnetic loss coefficient is secured on both surfaces of the substrate to reduce the effect of unwanted propagation modes. One of these propagation modes includes energy that propagates up the probe substrate and reflects off of the probe mounting block and propagates back down the substrate producing undesired distortions of the measured signals.
Probes allow relatively accurate on-wafer measurements of very small devices, such as transistors, inductors, capacitors, resistors, and the like at frequencies from direct current to hundreds of giga-hertz. Relatively accurate measurements can be made using one or more such probes connected to a vector network analyzer and then calibrating the system using a calibration substrate. The calibration substrate has various types of planar calibration elements formed on it, such as Line-Reflect-Line (LRL) calibration elements, Line-Reflect-Match (LRM) calibration elements, Open-Short-Load-Thru (OSL-T) calibration elements, and the like. Deviations from the ideal response of the probe/calibration substrate combination are stored in the network analyzer and software algorithms are typically used to compensate for these detected deviations as well as the non-ideal response of the network analyzer and the interface to the probe.
The calibration substrate is positioned on a conductive chuck and is typically maintained in position by a vacuum. The conductive chuck acts as a ground plane for the undesired microstrip modes when a signal is applied through the probe. In addition to the undesired microstrip modes, undesirable surface wave modes propagate through the substrate. Quartz spacers have been placed under the calibration substrate to reduce the parasitic modes generated in the calibration substrate. However, even with quartz spacers the parasitic modes still produce resonances, such as in the incident to reflected signal ratio as measured by the network analyzer.
Unsuccessful attempts have been made to reduce the surface wave modes on the calibration substrate by locating a limited amount of lossy material, such as nichrome (nickel chromium alloy), along the opposing edges of the calibration elements. However, the dimension of the nichrome material is much shorter than the wavelength of the signal being coupled into the calibration element. Therefore, it has little effect on surface wave modes which propagate along the bottom surface of the substrate. Additionally, it has little effect on the microstrip modes generated by the conductive chuck acting as a ground plane for the calibration elements.
Referring to FIG. 1, a cross-sectional view of the coplanar transmission structure 10 described in U.S. Pat. No. 5,225,796 is shown. The coplanar transmission structure 10 includes a substrate 12 having a coplanar transmission line 14, shown as a coplanar transmission waveguide, formed on one surface thereof. The substrate 12 is formed from a dielectric material, such as alumina or sapphire. The coplanar transmission line 14 may also be a coplanar stripline, as in FIG. 2. The coplanar transmission waveguide 14 includes a center strip 16 with two ground planes 18 and 20 located parallel to and in the plane of the center strip 16. The coplanar transmission line 14 defines the electromagnetic mode of radiation propagating along the transmission line 14, such as a quasi-TEM mode of propagation. The opposite surface of the substrate 12 has a layer of lossy resistive material 22, such as nichrome, tantalum nitride, or the like formed thereon.
The use of a lossy resistive material tends to attenuate the parasitic evanescent or propagating electromagnetic modes of the coplanar transmission structure. FIG. 2A shows a plan view of a coplanar transmission structure having an asymmetrical coplanar stripline 24 formed on one surface of a sapphire substrate 26. A layer of nichrome 28 is deposited adjacent and connected to the ground of the stripline 27. The cross-sectional view of FIG. 2B shows another approach to adding lossy material, with the layer of nichrome 30 formed on the opposite surface of the substrate 26.
Unfortunately, the structures shown in FIGS. 1 and 2 tend to result in relatively distorted signals over a large range of frequencies. The distortion results from undesirable modes propagating within the substrate. The precise source of the undesirable modes is unknown which results in difficulty in reducing the undesirable modes. The distortion levels are sufficiently large that for very accurate measurements the calibration substrate is simply ineffective.
A calibration substrate available from Cascade Microtech of Beaverton, Oreg. includes a set of calibration structures. Referring to FIG. 3, the calibration structures include a set of conductive members 54 supported by the substrate and spatially arranged across the substrate. Similar conductive members are aligned in an array orientation. To effectively increase the frequency response and smooth out the frequency response of the microstrip mode of the conductive members to the base of the substrate, a small portion of resistive material 56 is located adjacent to the end of each of the conductive members. The wider conductive members are approximately 525 microns wide and the thinner conductive members are approximately 100 microns wide, with a spacing of approximately 750 microns between conductive material columns. The resistive material is approximately 150 microns in length and has a width equal to that of the conductive material. The conductive members are approximately 1400 microns in length. The column of conductive members 60 are for open calibration tests, the column of conductive member 62 are for load calibration tests, the column of conductive members 64 are for short calibration tests, the column of conductive members 66 are for through conductive tests, and the column of conductive members 68 are for loop back through conductive tests.
While providing an improved measurement accuracy, the resulting structure includes a resonance at approximately 33 giga-hertz having a magnitude of approximately 0.10-0.15 dB deviation from the ideal (0 dB) when measuring a short calibration structure (S11 measurement), as shown in FIG. 4. The S11 magnitude in dB is calculated as 20*log(x), where x is the magnitude of the return signal with the input normalized to 1. This resonance makes probing of semiconductors more difficult in the general range of 33 giga-hertz because it is difficult to calibrate the system. In the case of a resonant system, Q is a measure of the sharpness of the resonant peak in the frequency response of the system and is inversely proportional to the damping in the system, and may be also considered the reactive portion over the resistive portion of the impedance causing the resonance. For example, Q=(center frequency in hertz)/(bandwidth (0.707 times down (3 dB reduction in magnitude) from the maximum magnitude at the center frequency)). Referring to FIG. 4, the Q factor of the impedance causing the 33 GHz resonance is approximately 22.
What is desired is calibration structures that reduce unwanted spurious modes.